James S Lai

Creep Behavior of Fiber-Reinforced Polymeric Composites: A Review of the Technical Literature:

This report provides a review of the technical literature related to the creep behavior of fiber reinforced polymer (FRP) composites. The review presented here was directed toward those papers that define the direction and line of thinking in the area of experimental techniques that may be candidates for the development of accelerated test methods to predict the long-term performance of FRP composite materials for highway structural applications. Linear and nonlinear viscoelastic theories as they relate to the modeling and prediction of the viscoelastic response of FRP materials under constant loads are included. Accelerated characterization techniques for the viscoelastic behavior of FRP composites including the use of elevated temperatures and frequency domain loading are reviewed. The effects of moisture and temperature on the creep behavior of composites are briefly considered. The interaction between creep behavior and fatigue behavior is also included in the discussion.

Three-Dimensional Image Analysis of Aggregate Particles from Orthogonal Projections

Digital image analysis provides the capability for rapid measurement of particle characteristics. When an image is captured and digitized, numerous measurements can be made in near real time for each particle. Usually, image analysis techniques treat particles as two-dimensional objects since only the two-dimensional projection of the particles is captured. In this study, three-dimensional analysis of aggregate particles that was performed by attaching aggregates in sample trays with two perpendicular faces is described. After the initial projected image of the aggregates is captured and measured, the sample trays are rotated 90 degrees so that the aggregates are now perpendicular to their original orientation and the dimensions of the aggregates in the new projected image are captured and measured. The long, intermediate, and short particle dimensions (dL, dI, and dS, respectively) provide direct measures of the flatness and elongation of the particles. Some other shape indexes can also be derived from t...

Use of Georgia loaded wheel tester to evaluate rutting of asphalt samples prepared by Superpave gyratory compactor

The Georgia loaded wheel tester (LWT) developed by the Georgia Department of Transportation has been used since 1985 in the laboratory during the design stage to evaluate rutting susceptibility of asphalt concrete mixtures. In the LWT testing, asphaltic concrete is subjected to an elevated temperature in a loaded wheel system under repetitive loading conditions, and the permanent deformation induced under the wheelpath is measured. This approach to assess rutting susceptibility was thought to be much more representative than the current test methods and can provide a fast and more accurate means of assessing rutting susceptibility of asphalt concrete under actual field conditions. The asphaltic beam samples used for the LWT testing are prepared by a rolling compaction machine. The new LWT developed in 1992 is described. To promote the concept of using Georgia LWT as a supplement to the Superpave Level 1 design procedure for evaluating permanent deformation of hot-mix asphalt (HMA), a test method utilizing...

Modifications of sulfur polymer cement (SPC) stabilization and solidification (S/S) process

This paper addresses the effectiveness of using sulfur polymer cement (SPC) as a binder to stabilize/solidify lead-contaminated soils. SPC, which has been used as a construction material because of its excellent resistance to acid and salt environments and its superior water tightness as compared with Portland cement concrete, has recently emerged as a possible alternative binder to stabilize/solidify soils contaminated with hazardous, low-level radioactive and mixed wastes. However, it was found that the use of SPC alone could not satisfactorily stabilize/solidify lead-contaminated soils. Nevertheless, it was shown that additives, such as sodium sulfide or sodium sulfite, could be used to greatly enhance the ability of SPC to react chemically with lead contaminants, and physicochemically to bind these compounds. These enable us significantly to lower the leachability (e.g. from 77.8 mg Pb/l to 1.28 mg Pb/l in EPA TCLP extract) of the SPC-treated wastes to the point where they can be recycled as some form of construction material.

Stabilization and solidification of lead in contaminated soils

Lead has been identified as one of the greatest threats to human health and is one of the common contaminants in many hazardous wastes. In this study, a surplus (waste) material, i.e., sulfur, was employed as a binder to stabilize/solidify lead contaminated soils. Soil samples were collected from a battery recovery plant, which had high levels of inorganic lead contaminant. Results obtained from the study indicated that sulfur binders can be used to stabilize/solidify inorganic lead contaminated soil which may or may not contain organic compounds. However, control samples, which used portland cement to solidify the same contaminated soils, showed that portland cement was also an effective binder. The potential applications of these solidified matrixes are also discussed. Due to the excellent physical, engineering and chemical leaching characteristics, sulfur solidified wastes could be used as construction fills, such as a subbase course in road pavement construction. Under some circumstances, use of the sulfur stabilization and solidification process will be a viable choice, especially where excess sulfur, recovered from various industrial desulfurization sites, becomes a waste product which requires disposal. The excess (waste) sulfur can be used as a stabilization agent for treating lead contaminated soil locally. Thus the two waste materials can be combined and converted into an environmentally stable material for recycling without having to be deposited in a landfill site. This by itself meets the requirements for being a sustainable technology as favored by the emerging world-wide trend of the economy for the future.

Development of Temperature-Effect Model for Predicting Rutting of Asphalt Mixtures Using Georgia Loaded Wheel Tester

Use of the Georgia loaded wheel tester (LWT) to evaluate rutting susceptibility of asphalt mixtures has gained acceptance by the asphalt paving industry. The test is typically conducted at 40°C for 8,000 cycles and the rut-depth value measured at the end of the test is compared with a maximum criteria of 5.0 mm or 7.5 mm to assess rutting susceptibility of the mixture. A temperature effect model (TEM) was developed using the LWT test data from seven asphalt mixtures. The TEM developed can be used to predict the rut-depth values of an asphalt mixture at different temperatures and number of loading cycles from the LWT performed on the asphalt mixture at one testing condition. The predicted rut-depth values from the TEM compared very closely with the measured values. For the five dense-graded hot-mix asphalt mixtures (HMA), only 7 out of 170 predicted rut-depth values deviated from the measured values by more than 0.8 mm. For the two stone-matrix asphalt mixtures (SMA), only 2 out of 64 predicted values deviated from the measured values by more than 0.8 mm. The TEM can be used to determine the equivalent rut-depth acceptance values at a lower number of rut-testing cycles, and thus can shorten the testing time for performing the LWT rutting susceptibility acceptance test. This can be useful for the field quality control of HMA. Using this predictive model, the LWT rutdepth acceptance criteria can be developed for asphalt mixtures at the temperatures more closely related to the actual pavement temperatures in the field.

NONINVASIVE MEASUREMENT OF PERMANENT STRAIN FIELD RESULTING FROM RUTTING IN ASPHALT CONCRETE

Asphalt concrete is a bonded granular material composed of aggregates and asphalt binder. The chemical, physical, and mechanical properties of these constituents are different. Under typical repeated tire loads, the aggregates do not deform but instead rigidly translate and rotate within the binder. Rutting is the surface manifestation of the internal-structure evolution in asphalt concrete under repeated wheel loading. A procedure to quantify the permanent strain field of asphalt concrete underlying the rutted zone produced by an accelerated wheel testing device is described. This new, noninvasive procedure has been automatically implemented in a computer program named MATCH, which was developed to compare the locations of particles before and after testing and therefore to permit the displacement of individual particles to be tracked. Pattern recognition based on each particle’s cross-sectional area, perimeter, and aspect ratio was used to match the particles. Application of the new procedure to measure the permanent strain field of an asphalt concrete specimen in a Georgia loaded wheel tester is described. The reliability of the procedure for automatically matching the particles in successive images was confirmed by manual procedures. It was found that the permanent strain and the mastic/solid area ratio evolution measured in this application demonstrated strong localism. Dilatancy and contraction were quite significant locally but insignificant globally. The strains in the mastic were much larger than the global or macrostrain. The proposed procedure has significance in the future study of micro-macro properties of bonded granular materials and how factors such as aggregate size, shape, and distribution affect the global response.

Prediction of Uniaxial Stress Relaxation from Creep of Nonlinear Viscoelastic Material

A method is described for predicting nonlinear stress relaxation from nonlinear creep data under constant uniaxial stress. This method utilizes as a first approximation an inversion of the function obtained from the multiple integral equation describing creep at constant stress. A correction procedure accounts for the variable stress during relaxation by employing the general multiple integral function with an assumption that the kernel functions containing mixed time parameters may be taken as products. The results computed from creep data with only one stage of corrections are in good agreement with relaxation experiments on a polyurethane tube.

Creep of 2618 Aluminum Under Side-Steps of Tension and Torsion and Stress Reversal Predicted by a Viscous-Viscoelastic Model

Abstract : Nonlinear constitutive equations were developed and used to predict the creep behavior of 2618-T61 Aluminum at 200 C (392 F) for combined tension and torsion stresses and under varying stress histories including side step stress changes and stress reversals. The constitutive equations consist of 5 components: linear elastic; time-independent plastic; nonlinear time-dependent plastic recoverable; nonlinear time-dependent nonrecoverable under positive stress; and nonlinear time-dependent nonrecoverable under negative stress. For time-dependent stress inputs, the modified superposition principle and strain hardening are used to describe the behavior of nonlinear time-dependent recoverable and nonlinear time-dependent nonrecoverable respectively. The theory which combines all these features, the viscous-viscoelastic theory, and other modified theories were used to predict from information from constant stress creep the creep behavior of 2618 aluminum under the above stress histories with very satisfactory agreement with the experimental results. (Author)

Creep of 2618 Aluminum Under Step Stress Changes Predicted by a Viscous-Viscoelastic Model

Abstract : Nonlinear constitutive equations are developed and used to predict from constant stress data the creep behavior of 2618 Aluminum at 200 C (392 F) for tension or torsion stresses under varying stress history including step-up, step-down, and reloading stress changes. The strain in the constitutive equation employed includes the following components: linear elastic, time-independent plastic, nonlinear time-dependent recoverable (viscoelastic), nonlinear time-dependent nonrecoverable (viscous) positive, and nonlinear time-dependent nonrecoverable (viscous) negative. The modified superposition principle, derived from the multiple integral representation, and strain hardening theory were used to represent the recoverable and nonrecoverable components, respectively, of the time-dependent strain in the constitutive equations. (Author)